Method and apparatus for performing inter-menb handover without senb change in wireless communication system

ABSTRACT

A method and apparatus for performing an inter-master eNodeB (MeNB) handover procedure without secondary eNB (SeNB) change in a wireless communication system is provided. A source MeNB transmits a handover request message including a first indication of a SeNB to a target MeNB. The first indication may correspond to a SeNB identifier (ID). Upon receiving the handover request message including the first indication, the target MeNB transmits a SeNB addition request message including a second indication of a UE, served by the SeNB, to the SeNB. The second indication may correspond to an old SeNB UE X2AP ID, which was allocated by the SeNB. Upon receiving the SeNB addition request message including the second indication, the SeNB may transmit a SeNB addition acknowledge message including a third indication of keeping bearers of the SeNB from the SeNB.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for performing an inter-mastereNodeB (MeNB) handover without secondary eNB (SeNB) change in a wirelesscommunication system.

Related Art

The 3GPP LTE is a technology for enabling high-speed packetcommunications. Many schemes have been proposed for the LTE objectiveincluding those that aim to reduce user and provider costs, improveservice quality, and expand and improve coverage and system capacity.The 3GPP LTE requires reduced cost per bit, increased serviceavailability, flexible use of a frequency band, a simple structure, anopen interface, and adequate power consumption of a terminal as anupper-level requirement.

Small cells using low power nodes are considered promising to cope withmobile traffic explosion, especially for hotspot deployments in indoorand outdoor scenarios. A low-power node generally means a node whosetransmission power is lower than macro node and base station (BS)classes, for example pico and femto evolved NodeB (eNB) are bothapplicable. Small cell enhancements for evolved UMTS terrestrial radioaccess (E-UTRA) and evolved UMTS terrestrial radio access network(E-UTRAN) will focus on additional functionalities for enhancedperformance in hotspot areas for indoor and outdoor using low powernodes.

One of potential solutions for small cell enhancement, dual connectivityhas been discussed. Dual connectivity is used to refer to operationwhere a given UE consumes radio resources provided by at least twodifferent network points connected with non-ideal backhaul. Furthermore,each eNB involved in dual connectivity for a UE may assume differentroles. Those roles do not necessarily depend on the eNB's power classand can vary among UEs. Dual connectivity may be one of potentialsolutions for small cell enhancement.

As the dual connectivity has been introduced, various scenarios forhandover may happen. Specifically, when one secondary eNB (SeNB) in dualconnectivity is shared by two master eNBs (MeNBs), inter-MeNB handoverwithout SeNB change may happen. A method for performing inter-MeNBhandover without SeNB effectively may be required.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for performing aninter-master eNodeB (MeNB) handover without secondary eNB (SeNB) changein a wireless communication system. The present invention provides amethod and apparatus for transmitting a handover request messageincluding an indication of a SeNB in dual connectivity. The presentinvention provides a method and apparatus for transmitting a SeNBaddition request message including an indication of a user equipment(UE) served by the SeNB.

In an aspect, a method for performing, by a source master eNodeB (MeNB)in a dual connectivity, a handover procedure in a wireless communicationsystem is provided. The method includes transmitting a handover requestmessage including an indication of a secondary eNodeB (SeNB) in dualconnectivity to a target MeNB in dual connectivity, and receiving ahandover request acknowledge message from the target MeNB.

The indication of the SeNB may correspond to a SeNB identifier (ID).

The indication of the SeNB may correspond to a list of candidate SeNBIDs.

The indication of the SeNB may correspond to a SeNB UE X2AP ID, whichwas allocated by the SeNB. The SeNB UE X2AP ID may be allocated by theSeNB before the handover procedure is triggered.

The handover request message may further include measurement results ofsecondary cell group (SCG) cells for the SeNB.

The handover request acknowledge message may include an indication ofkeeping bearers of the SeNB.

A UE may be connected to both the source MeNB and the SeNB currently,and the UE may be to be handed over to the target MeNB by the handoverprocedure without change of the SeNB.

In another aspect, a method for performing, by a target master eNodeB(MeNB) in a dual connectivity, a handover procedure in a wirelesscommunication system is provided. The method includes receiving ahandover request message including a first indication of a secondaryeNodeB (SeNB) in dual connectivity from a source MeNB in dualconnectivity, and transmitting a SeNB addition request message includinga second indication of a UE, served by the SeNB, to the SeNB.

The second indication of the UE may correspond to an old SeNB UE X2APID, which was allocated by the SeNB.

The SeNB addition request message may further include a source MeNB ID.

The method may further include receiving a SeNB addition acknowledgemessage including a third indication of keeping bearers of the SeNB fromthe SeNB. The third indication of keeping bearers of the SeNB mayindicate whether original SeNB bearers are all accepted or partiallyaccepted or indicates which specific SeNB bearer is accepted or not.

Inter-MeNB handover without SeNB change can be performed effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC.

FIG. 3 shows a block diagram of a user plane protocol stack of an LTEsystem.

FIG. 4 shows a block diagram of a control plane protocol stack of an LTEsystem.

FIG. 5 shows an example of a physical channel structure.

FIG. 6 shows radio protocol architecture for dual connectivity.

FIG. 7 shows C-plane connectivity of eNBs involved in dual connectivityfor a certain UE.

FIG. 8 shows U-plane connectivity of eNBs involved in dual connectivityfor a certain UE.

FIG. 9 shows an example of U-plane architecture for dual connectivity.

FIG. 10 shows another example of U-plane architecture for dualconnectivity.

FIG. 11 shows an example of an X2 handover procedure for dualconnectivity enhancement.

FIG. 12 shows an example of a SeNB addition procedure.

FIG. 13 shows an example of a deployment scenario with a common SeNBshared by two MeNBs.

FIG. 14 shows a method for exchanging information about whether a commonSeNB exists between MeNBs according to an embodiment of the presentinvention.

FIG. 15 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

FIG. 16 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

FIG. 17 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

FIG. 18 shows a method for performing, by a target MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention.

FIG. 19 shows a method for performing, by a source MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention.

FIG. 20 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

FIG. 21 shows a method for performing, by a source MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention.

FIG. 22 shows a method for performing, by a target MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention.

FIG. 23 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

FIG. 24 shows an example of a deployment scenario with two SeNBs and twoMeNBs.

FIG. 25 shows a method for performing an inter-MeNB handover accordingto an embodiment of the present invention.

FIG. 26 shows a wireless communication system to implement an embodimentof the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802-20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is an evolution of IEEE 802.16e, and provides backwardcompatibility with an IEEE 802.16-based system. The UTRA is a part of auniversal mobile telecommunication system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of anevolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA indownlink and uses the SC-FDMA in uplink. LTE-advance (LTE-A) is anevolution of the 3GPP LTE.

For clarity, the following description will focus on the LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), anaccess point, etc. One eNB 20 may be deployed per cell.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) and a systemarchitecture evolution (SAE) gateway (S-GW). The MME/S-GW 30 may bepositioned at the end of the network and connected to an externalnetwork. For clarity, MME/S-GW 30 will be referred to herein simply as a“gateway,” but it is understood that this entity includes both the MMEand S-GW.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), packet data network (PDN)gateway (P-GW) and S-GW selection, MME selection for handovers with MMEchange, serving GPRS support node (SGSN) selection for handovers to 2Gor 3G 3GPP access networks, roaming, authentication, bearer managementfunctions including dedicated bearer establishment, support for publicwarning system (PWS) (which includes earthquake and tsunami warningsystem (ETWS) and commercial mobile alert system (CMAS)) messagetransmission. The S-GW host provides assorted functions includingper-user based packet filtering (by e.g., deep packet inspection),lawful interception, UE Internet protocol (IP) address allocation,transport level packet marking in the DL, UL and DL service levelcharging, gating and rate enforcement, DL rate enforcement based onaccess point name aggregate maximum bit rate (APN-AMBR).

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 is connected to the eNB 20 via a Uu interface. The eNBs 20 areconnected to each other via an X2 interface. Neighboring eNBs may have ameshed network structure that has the X2 interface. A plurality of nodesmay be connected between the eNB 20 and the gateway 30 via an S1interface.

FIG. 2 shows a block diagram of architecture of a typical E-UTRAN and atypical EPC. Referring to FIG. 2, the eNB 20 may perform functions ofselection for gateway 30, routing toward the gateway 30 during a radioresource control (RRC) activation, scheduling and transmitting of pagingmessages, scheduling and transmitting of broadcast channel (BCH)information, dynamic allocation of resources to the UEs 10 in both ULand DL, configuration and provisioning of eNB measurements, radio bearercontrol, radio admission control (RAC), and connection mobility controlin LTE_ACTIVE state. In the EPC, and as noted above, gateway 30 mayperform functions of paging origination, LTE_IDLE state management,ciphering of the user plane, SAE bearer control, and ciphering andintegrity protection of NAS signaling.

FIG. 3 shows a block diagram of a user plane protocol stack of an LTEsystem. FIG. 4 shows a block diagram of a control plane protocol stackof an LTE system. Layers of a radio interface protocol between the UEand the E-UTRAN may be classified into a first layer (L1), a secondlayer (L2), and a third layer (L3) based on the lower three layers ofthe open system interconnection (OSI) model that is well-known in thecommunication system.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel. Databetween the MAC layer and the PHY layer is transferred through thetransport channel. Between different PHY layers, i.e., between a PHYlayer of a transmission side and a PHY layer of a reception side, datais transferred via the physical channel.

A MAC layer, a radio link control (RLC) layer, and a packet dataconvergence protocol (PDCP) layer belong to the L2. The MAC layerprovides services to the RLC layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides data transferservices on logical channels. The RLC layer supports the transmission ofdata with reliability. Meanwhile, a function of the RLC layer may beimplemented with a functional block inside the MAC layer. In this case,the RLC layer may not exist. The PDCP layer provides a function ofheader compression function that reduces unnecessary control informationsuch that data being transmitted by employing IP packets, such as IPv4or IPv6, can be efficiently transmitted over a radio interface that hasa relatively small bandwidth.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer controls logical channels, transportchannels, and physical channels in relation to the configuration,reconfiguration, and release of radio bearers (RBs). The RB signifies aservice provided the L2 for data transmission between the UE andE-UTRAN.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid ARQ (HARQ). The PDCP layer (terminatedin the eNB on the network side) may perform the user plane functionssuch as header compression, integrity protection, and ciphering.

Referring to FIG. 4, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The RRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

FIG. 5 shows an example of a physical channel structure. A physicalchannel transfers signaling and data between PHY layer of the UE and eNBwith a radio resource. A physical channel consists of a plurality ofsubframes in time domain and a plurality of subcarriers in frequencydomain. One subframe, which is 1 ms, consists of a plurality of symbolsin the time domain. Specific symbol(s) of the subframe, such as thefirst symbol of the subframe, may be used for a physical downlinkcontrol channel (PDCCH). The PDCCH carries dynamic allocated resources,such as a physical resource block (PRB) and modulation and coding scheme(MCS).

A DL transport channel includes a broadcast channel (BCH) used fortransmitting system information, a paging channel (PCH) used for paginga UE, a downlink shared channel (DL-SCH) used for transmitting usertraffic or control signals, a multicast channel (MCH) used for multicastor broadcast service transmission. The DL-SCH supports HARQ, dynamiclink adaptation by varying the modulation, coding and transmit power,and both dynamic and semi-static resource allocation. The DL-SCH alsomay enable broadcast in the entire cell and the use of beamforming.

A UL transport channel includes a random access channel (RACH) normallyused for initial access to a cell, a uplink shared channel (UL-SCH) fortransmitting user traffic or control signals, etc. The UL-SCH supportsHARQ and dynamic link adaptation by varying the transmit power andpotentially modulation and coding. The UL-SCH also may enable the use ofbeamforming.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting multimedia broadcast multicast services(MBMS) control information from the network to a UE. The DCCH is apoint-to-point bi-directional channel used by UEs having an RRCconnection that transmits dedicated control information between a UE andthe network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RRC state indicates whether an RRC layer of the UE is logicallyconnected to an RRC layer of the E-UTRAN. The RRC state may be dividedinto two different states such as an RRC idle state (RRC_IDLE) and anRRC connected state (RRC_CONNECTED). In RRC_IDLE, the UE may receivebroadcasts of system information and paging information while the UEspecifies a discontinuous reception (DRX) configured by NAS, and the UEhas been allocated an identification (ID) which uniquely identifies theUE in a tracking area and may perform public land mobile network (PLMN)selection and cell re-selection. Also, in RRC_IDLE, no RRC context isstored in the eNB.

In RRC_CONNECTED, the UE has an E-UTRAN RRC connection and a context inthe E-UTRAN, such that transmitting and/or receiving data to/from theeNB becomes possible. Also, the UE can report channel qualityinformation and feedback information to the eNB. In RRC_CONNECTED, theE-UTRAN knows the cell to which the UE belongs. Therefore, the networkcan transmit and/or receive data to/from UE, the network can controlmobility (handover and inter-radio access technologies (RAT) cell changeorder to GSM EDGE radio access network (GERAN) with network assistedcell change (NACC)) of the UE, and the network can perform cellmeasurements for a neighboring cell.

In RRC_IDLE, the UE specifies the paging DRX cycle. Specifically, the UEmonitors a paging signal at a specific paging occasion of every UEspecific paging DRX cycle. The paging occasion is a time interval duringwhich a paging signal is transmitted. The UE has its own pagingoccasion. A paging message is transmitted over all cells belonging tothe same tracking area. If the UE moves from one tracking area (TA) toanother TA, the UE will send a tracking area update (TAU) message to thenetwork to update its location.

Overall architecture and network interface for dual connectivity (DC) isdescribed. It may be referred to 3GPP TR 36.842 V12.0.0 (2013 December).The E-UTRAN may support dual connectivity operation whereby a multipleRX/TX UE in RRC_CONNECTED is configured to utilize radio resourcesprovided by two distinct schedulers, located in two eNBs connected via anon-ideal backhaul over the X2 interface. The overall E-UTRANarchitecture described in FIG. 1 is applicable for dual connectivity aswell. Two different roles may be assumed to eNBs involved in dualconnectivity for a certain UE: an eNB may either act as a master eNB(MeNB) or as a secondary eNB (SeNB). The MeNB is the eNB whichterminates at least S1-MME in dual connectivity. The SeNB is the eNBthat is providing additional radio resources for the UE but is not theMeNB in dual connectivity. In dual connectivity a UE is connected to oneMeNB and one SeNB.

FIG. 6 shows radio protocol architecture for dual connectivity. In DC,the radio protocol architecture that a particular bearer uses depends onhow the bearer is setup. Three alternatives exist, master cell group(MCG) bearer, secondary cell group (SCG) bearer and split bearer.Referring to FIG. 6, those three alternatives are depicted, i.e. inorder of the MCG bearer, split bearer and SCG bearer from left to right.The MCG bearer is a bearer whose radio protocols are only located in theMeNB to use MeNB resources only in dual connectivity. The SCG bearer isa bearer whose radio protocols are only located in the SeNB to use SeNBresources in dual connectivity. The split bearer is a bearer whose radioprotocols are located in both the MeNB and the SeNB to use both MeNB andSeNB resources in dual connectivity. Signaling radio bearers (SRBs) arealways of the MCG bearer and therefore only use the radio resourcesprovided by the MeNB. The MCG is a group of serving cells associatedwith the MeNB, comprising of the primary cell (PCell) and optionally oneor more secondary cells (SCells) in dual connectivity. The SCG is agroup of serving cells associated with the SeNB, comprising of primarySCell (PSCell) and optionally one or more SCells in dual connectivity.DC may also be described as having at least one bearer configured to useradio resources provided by the SeNB.

FIG. 7 shows C-plane connectivity of eNBs involved in dual connectivityfor a certain UE. Inter-eNB control plane signaling for dualconnectivity is performed by means of X2 interface signaling. Controlplane signaling towards the MME is performed by means of S1 interfacesignaling. There is only one S1-MME connection per UE between the MeNBand the MME. Each eNB should be able to handle UEs independently, i.e.provide the PCell to some UEs while providing SCell(s) for SCG toothers. Each eNB involved in dual connectivity for a certain UE owns itsradio resources and is primarily responsible for allocating radioresources of its cells, respective coordination between MeNB and SeNB isperformed by means of X2 interface signaling. Referring to FIG. 7, theMeNB is C-plane connected to the MME via S1-MME, the MeNB and the SeNBare interconnected via X2-C.

FIG. 8 shows U-plane connectivity of eNBs involved in dual connectivityfor a certain UE. U-plane connectivity depends on the bearer optionconfigured. For MCG bearers, the MeNB is U-plane connected to the S-GWvia S1-U, the SeNB is not involved in the transport of user plane data.For split bearers, the MeNB is U-plane connected to the S-GW via S1-Uand in addition, the MeNB and the SeNB are interconnected via X2-U. ForSCG bearers, the SeNB is directly connected with the S-GW via S1-U. Ifonly MCG and split bearers are configured, there is no S1-U terminationin the SeNB.

FIG. 9 shows an example of U-plane architecture for dual connectivity.U-plane architecture for dual connectivity shown in FIG. 9 is thecombination of S1-U that terminates in SeNB and independent PDCPs (nobearer split).

FIG. 10 shows another example of U-plane architecture for dualconnectivity. U-plane architecture for dual connectivity shown in FIG.10 is the combination of S1-U that terminates in MeNB, bearer split inMeNB, and independent RLCs for split bearers.

The corresponding UE architecture may be also changed to support the newfeature.

FIG. 11 shows an example of an X2 handover procedure for dualconnectivity enhancement. FIG. 11 shows an example signaling flow for aMeNB to eNB change procedure, which is used to transfer context datafrom a source MeNB (S-MeNB)/SeNB to a target eNB (T-eNB).

In step S1110, the source MeNB starts the MeNB to eNB change procedureby initiating the X2 handover preparation procedure, i.e. transmittingthe Handover Request message to the target eNB. The source MeNB includesthe SCG configuration in the HandoverPreparationlnformation. Table 1shows the Handover Request message. This message is sent by the sourceeNB to the target eNB to request the preparation of resources for ahandover.

TABLE 1 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject Old eNB UE M eNB UE X2AP Allocated at the YES reject X2AP ID IDsource eNB 9.2.24 Cause M 9.2.6 YES ignore Target Cell ID M ECGI YESreject 9.2.14 GUMMEI M 9.2.16 YES reject UE Context 1 YES rejectInformation >MME UE S1AP M INTEGER MME UE S1AP ID — — ID (0 . . . 2³²−1) allocated at the MME >UE Security M 9.2.29 — — Capabilities >ASSecurity M 9.2.30 — — Information >UE Aggregate M 9.2.12 — — Maximum BitRate >Subscriber O 9.2.25 — — Profile ID for RAT/Frequencypriority >E-RABs To Be 1 — — Setup List >>E-RABs To 1 . . . <max noofBearers> EACH ignore Be Setup Item >>>E-RAB ID M 9.2.23 — — >>>E-RAB M9.2.9 Includes necessary — — Level QoS QoS parameters Parameters >>>DL O9.2.5 — — Forwarding >>>UL GTP M GTP Tunnel SGW endpoint of — — TunnelEndpoint 9.2.1 the S1 transport Endpoint bearer. For delivery of ULPDUs. >RRC Context M OCTET Includes the RRC — — STRING HandoverPreparation Information message as defined in subclause 10.2.2 of TS36.331 [9] >Handover O 9.2.3 — — Restriction List >Location O 9.2.21Includes the — — Reporting necessary Information parameters for locationreporting >Management O 9.2.59 YES ignore Based MDT Allowed >ManagementO MDT PLMN YES ignore Based MDT List PLMN List 9.2.64 UE History M9.2.38 Same definition as YES ignore Information in TS 36.413 [4] TraceActivation O 9.2.2 YES ignore SRVCC Operation O 9.2.33 YES ignorePossible CSG Membership O 9.2.52 YES reject Status Mobility O BIT STRINGInformation related YES ignore Information (SIZE (32)) to the handover;the source eNB provides it in order to enable later analysis of theconditions that led to a wrong HO. Masked IMEISV O 9.2.69 YES ignore UEHistory O OCTET VisitedCellInfoList YES ignore Information from STRINGcontained in the the UE UEInformationResponse message (TS 36.331 [9])Expected UE O 9.2.70 YES ignore Behaviour

In step S1101, the target eNB transmits the Handover Request Acknowledgemessage to the source MeNB. The target eNB includes the field inhandover command which releases SCG configuration, and may also provideforwarding addresses to the source MeNB. The addition of a SeNB can beinitiated only after completing handover. Table 2 shows the HandoverRequest Acknowledge message. This message is sent by the target eNB toinform the source eNB about the prepared resources at the target.

TABLE 2 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject Old eNB UE M eNB UE X2AP Allocated at the YES ignore X2AP ID IDsource eNB 9.2.24 New eNB UE M eNB UE X2AP Allocated at the YES ignoreX2AP ID ID target eNB 9.2.24 E-RABs Admitted 1 YES ignore List > E-RABs1 . . . <max noof Bearers> EACH ignore Admitted Item >>E-RAB ID M 9.2.23— — >>UL GTP Tunnel O GTP Tunnel Identifies the X2 — — Endpoint Endpoint9.2.1 transport bearer used for forwarding of UL PDUs >>DL GTP Tunnel OGTP Tunnel Identifies the X2 — — Endpoint Endpoint 9.2.1 transportbearer. used for forwarding of DL PDUs E-RABs Not O E-RAB List A valuefor E-RAB YES ignore Admitted List 9.2.28 ID shall only be present oncein E-RABs Admitted List IE and in E- RABs Not Admitted List IE. TargeteNB To M OCTET Includes the RRC YES ignore Source eNB STRING E-UTRAHandover Transparent Command message Container as defined in subclause10.2.2 in TS 36.331 [9] Criticality O 9.2.7 YES ignore Diagnostics

In step S1110, if the allocation of target eNB resources was successful,the MeNB initiates the release of the source SeNB resources towards thesource SeNB by transmitting the SeNB Release Request message. If dataforwarding is needed, the MeNB provides data forwarding addresses to thesource SeNB. Either direct data forwarding or indirect data forwardingis used for SCG bearer. Only indirect data forwarding is used for splitbearer. Reception of the SeNB Release Request message triggers thesource SeNB to stop providing user data to the UE and, if applicable, tostart data forwarding. Table 3 shows the SeNB Release Request message.This message is sent by the MeNB to the SeNB to request the release ofresources.

TABLE 3 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESignore MeNB UE X2AP M eNB UE X2AP Allocated at the YES reject ID ID MeNB9.2.24 SeNB UE X2AP O eNB UE X2AP Allocated at the YES reject ID ID SeNB9.2.24 Cause O 9.2.6 YES ignore E-RABs To Be 0 . . . 1 — — ReleasedList > E-RABs To 1 . . . <max noof Bearers> EACH ignore Be ReleasedItem >>CHOICE M Bearer Option >>>SCG Bearer >>>>E-RAB ID M 9.2.23 —— >>>>UL O GTP Tunnel Identifies the X2 — — Forwarding GTP Endpoint9.2.1 transport bearer Tunnel Endpoint used for forwarding of ULPDUs >>>>DL O GTP Tunnel Identifies the X2 — — Forwarding GTP Endpoint9.2.1 transport bearer. Tunnel Endpoint used for forwarding of DLPDUs >>>Split Bearer >>>>E-RAB ID M 9.2.23 — — >>>>DL O GTP TunnelIdentifies the X2 — — Forwarding GTP Endpoint 9.2.1 transport bearer.Tunnel Endpoint used for forwarding of DL PDUs

In step S1120, the MeNB triggers the UE to apply the new configurationby transmitting the RRCConnectionReconfiguration message to the UE. Uponreceiving the new configuration, the UE releases the entire SCGconfiguration.

The UE synchronizes to the target eNB. In step S1130, the UE performsrandom access procedure with the target eNB. In step S1131, the UEtransmits the RRCConnectionReconfigurationComplete message to the targeteNB.

In step S1140, sequence number (SN) Status Transfer message may beexchanged between the source MeNB and SeNB/target eNB. In step S1141,data forwarding from the SeNB takes place for E-UTRAN radio accessbearers (E-RABs) configured with the SCG bearer option. It may start asearly as the source SeNB receives the SeNB Release Request message fromthe MeNB.

The target eNB initiates the S1 path switch procedure. In step S1150,the target eNB may transmit the Path Switch Request message to the MME.In step S1151, the MME may perform the bearer modification procedurewith the S-GW. In step S1152, the end marker packet may be transmittedfrom the S-GW, via the source MeNB, to the target eNB. In step S1153,the new packets may be transmitted from the S-GW to the target eNB. Instep S1154, the MME may transmit Path Switch Request Acknowledge messageto the target eNB.

In step S1160, the target eNB initiates the UE context release proceduretowards the source MeNB.

In step S1161, upon reception of the UE Context Release message, theSeNB can release radio and C-plane related resource associated to the UEcontext. Any ongoing data forwarding may continue.

FIG. 12 shows an example of a SeNB addition procedure. The SeNB additionprocedure is initiated by the MeNB and is used to establish a UE contextat the SeNB in order to provide radio resources from the SeNB to the UE.This procedure is used to add at least the first cell (PSCell) of theSCG.

In step S1200, the MeNB decides to request the SeNB to allocate radioresources for a specific E-RAB, indicating E-RAB characteristics (E-RABparameters, TNL address information corresponding to the UP option). Inaddition, MeNB indicates within SCG-ConfigInfo the MCG configuration(including security algorithm for SCG bearer) and the entire UEcapabilities for UE capability coordination to be used as basis for thereconfiguration by the SeNB, but does not include SCG configuration. TheMeNB can provide the latest measurement results for the SCG cell(s)requested to be added. The SeNB may reject the request. Table 4 showsthe SeNB Addition Request message. This message is sent by the MeNB tothe SeNB to request the preparation of resources for dual connectivityoperation for a specific UE.

TABLE 4 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject MeNB UE X2AP M eNB UE X2AP Allocated at the YES reject ID ID MeNB9.2.24 UE Security C- 9.2.29 YES reject Capabilities ifSCG Bearer OptionSeNB Security C- 9.2.72 The S-KeNB which YES reject Key ifSCG isprovided by the Bearer MeNB, see TS Option 33.401 [18]. SeNB UE M UEAggregate The UE Aggregate YES reject Aggregate Maximum Bit Maximum BitRate Maximum Bit Rate Rate is split into MeNB 9.2.12 UE AggregateMaximum Bit Rate and SeNB UE Aggregate Maximum Bit Rate which areenforced by MeNB and SeNB respectively. Serving PLMN O PLMN Identity Theserving PLMN YES ignore 9.2.4 of the SCG in the SeNB. E-RABs To Be 1 YESreject Added List >E-RAB s To Be 1 . . . <max noof Bearers> EACH rejectAdded Item >>CHOICE M Bearer Option >>>SCG Bearer >>>>E-RAB M 9.2.23 — —ID >>>>E-RAB M 9.2.9 Includes necessary — — Level QoS QoS parametersParameters >>>>DL O 9.2.5 — — Forwarding >>>>S1 UL M GTP Tunnel SGWendpoint of — — GTP Tunnel Endpoint 9.2.1 the S1 transport Endpointbearer. For delivery of UL PDUs. >>>Split Bearer >>>>E-RAB M 9.2.23 — —ID >>>>E-RAB M 9.2.9 Includes necessary — — Level QoS QoS parametersParameters >>>>MeNB M GTP Tunnel MeNB endpoint of — — GTP TunnelEndpoint 9.2.1 the X2 transport Endpoint bearer. For delivery of ULPDUs. MeNB to SeNB M OCTET Includes the SCG- YES reject Container STRINGConfigInfo message as defined in TS 36.331 [9]

In step S1201, if the radio resource management (RRM) entity in the SeNBis able to admit the resource request, it allocates respective radioresources and, dependent on the bearer option, respective transportnetwork resources. The SeNB triggers random access so thatsynchronization of the SeNB radio resource configuration can beperformed. The SeNB provides the new radio resource of SCG in SCG-Configto the MeNB. For SCG bearers, together with S1 DL TNL addressinformation for the respective E-RAB and security algorithm, for splitbearers X2 DL TNL address information.

In step S1210, if the MeNB endorses the new configuration, the MeNBsends the RRCConnectionReconfiguration message to the UE including thenew radio resource configuration of SCG according to the SCG-Config.

In step S1211, the UE applies the new configuration and replies withRRCConnectionReconfigurationComplete message. In case the UE is unableto comply with (part of) the configuration included in theRRCConnectionReconfiguration message, it performs the reconfigurationfailure procedure.

In step S1220, the MeNB informs the SeNB that the UE has completed thereconfiguration procedure successfully.

In step S1230, the UE performs synchronization towards the PSCell of theSeNB. The order the UE sends the RRCConnectionReconfigurationCompletemessage and performs the random access procedure towards the SCG is notdefined. The successful RA procedure towards the SCG is not required fora successful completion of the RRC Connection Reconfiguration procedure.

In step S1240 and S1241, in case SCG bearers, and dependent on thebearer characteristics of the respective E-RAB, the MeNB may takeactions to minimize service interruption due to activation of dualconnectivity (Data forwarding, SN Status transfer).

In step S1250, for SCG bearers, the update of the UP path towards theEPC is performed.

FIG. 13 shows an example of a deployment scenario with a common SeNBshared by two MeNBs. Referring to FIG. 13, a UE is receiving dualconnectivity service via bearer 1 from MeNB 1, and via bearer 2 fromSeNB. The UE may be handed over to the neighbor MeNB, i.e. MeNB 2, whichshares the SeNB with the MeNB 1.

If the current X2 handover procedure shown in FIG. 11 is applied to thedeployment scenario shown in FIG. 13, the SeNB shared by two MeNBs hasto be released from MeNB 1 first, and the target MeNB, i.e. MeNB 2, mayadd the SeNB again after X2 handover. That is, the SeNB additionprocedure shown in FIG. 12 has to be performed again. This may not bethe optimized procedure.

In order to solve the problem described above, various methods forperforming an inter-MeNB handover without SeNB change are describedbelow according to embodiments of the present invention.

FIG. 14 shows a method for exchanging information about whether a commonSeNB exists between MeNBs according to an embodiment of the presentinvention.

In step S1400, the eNB 1 transmits the X2 Setup Request message, whichincludes a specific indicator for specific serving cells of the eNB 1,to the eNB 2. The specific indicator may contain cell IDs of thespecific overlapping serving cells. The specific indicator may furtherindicate that the specific serving cells provide dual connectivityservice. The specific serving cells may correspond to potential SeNBsshared with the eNB 2.

In step S1410, the eNB 2 replies to the eNB 1 by transmitting the X2Setup Response message, which also includes a specific indicator forspecific serving cells of the eNB 2. The specific indicator may containcell IDs of the specific overlapping serving cells. The specificindicator may further indicate that the specific serving cells providedual connectivity service. The specific serving cells may correspond topotential SeNBs shared with the eNB 1.

When the eNB2 or eNB1 receives the X2 Setup Request/Response messagewith the specific indicator, it may check its own serving cells andcompare them with the specific serving cells indicated by the specificindicator. Accordingly, the eNB 2 or eNB 1 may identify which cell is acommon cell shared by neighbor eNBs and provides dual connectivityservice.

FIG. 15 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention. In thisembodiment, it may be assumed that the SeNB bearers (or, SeNB E-RABs)are not all kept during the handover. At first, the UE (not described inFIG. 15) may transmit the measurement report to the source MeNB, whichmakes a handover decision to the target MeNB for the UE. The source MeNBmay also know that the UE is receiving a certain service from the SeNB,which is commonly controlled by the source MeNB and the target MeNB.

In step S1500, the source MeNB transmits the Handover Request message,which includes a first indicator, to target MeNB. The first indicatormay be indication of SeNB bearers and/or MeNB bearers. By the firstindicator, the target MeNB may differentiate the E-RABs To Be Setup Listfor the SeNB (i.e. SeNB bearers) and the E-RABs To Be Setup List for theMeNB (i.e. MeNB bearers). Further, by the first indicator, the targetMeNB may know that the source MeNB has intention to keep the SeNBservice/bearer. In this case, the first indicator may be realized bySeNB UE X2AP ID, which was allocated by the SeNB before handover istriggered. Further, the first indicator may be an indicator of the SeNBto indicate the target MeNB which SeNB service/bearer is kept for thisthe UE. In this case, the first indicator may be a SeNB (cell) ID. Or,the first indicator may be a list of potential candidate SeNBs for thetarget MeNB to select.

Further, the Handover Request message may include assistant information,i.e. measurement information, for the target MeNB to select the targetSeNB. The measurement information may be contained in an independent IEin the Handover Request message. For example, the measurementinformation may be contained in MeNB to SeNB Container includingSCG-ConfigInfo, as measResultServCellListSCG indicating measurementresults of SCG (serving) cells. Or, the measurement information may becontained in RRC Context IE in the Handover Request Message.

In other words, the first indicator may help the target MeNB to make adecision on how to keep the SeNB. Table 5 shows an example of theHandover Request message according to an embodiment of the presentinvention.

TABLE 5 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject Old eNB UE M eNB UE X2AP Allocated at the YES reject X2AP ID IDsource eNB 9.2.24 Cause M 9.2.6 YES ignore Target Cell ID M ECGI YESreject 9.2.14 GUMMEI M 9.2.16 YES reject UE Context 1 YES rejectInformation >MME UE S1AP M INTEGER MME UE S1AP ID — — ID (0 . . . 2³²−1) allocated at the MME >UE Security M 9.2.29 — — Capabilities >ASSecurity M 9.2.30 — — Information >UE Aggregate M 9.2.12 — — Maximum BitRate >Subscriber O 9.2.25 — — Profile ID for RAT/Frequencypriority >E-RABs To Be 1 — — Setup List >>E-RABs To 1 . . . <max noofBearers> EACH ignore Be Setup Item >>>E-RAB ID M 9.2.23 — — >>>E-RAB M9.2.9 Includes necessary — — Level QoS QoS parameters Parameters >>>DL O9.2.5 — — Forwarding >>>UL GTP M GTP Tunnel SGW endpoint of — — TunnelEndpoint 9.2.1 the S1 transport Endpoint bearer. For delivery of ULPDUs. >RRC Context M OCTET Includes the RRC — — STRING HandoverPreparation Information message as defined in subclause 10.2.2 of TS36.331 [9] It may also contain the measurement results of SCG (serving)cells, details are defined in TS 36.331 >Handover O 9.2.3 — —Restriction List >Location O 9.2.21 Includes the — — Reporting necessaryInformation parameters for location reporting >Management O 9.2.59 YESignore Based MDT Allowed >Management O MDT PLMN YES ignore Based MDTList PLMN List 9.2.64 UE History M 9.2.38 Same definition as YES ignoreInformation in TS 36.413 [4] Trace Activation O 9.2.2 YES ignore SRVCCOperation O 9.2.33 YES ignore Possible CSG Membership O 9.2.52 YESreject Status Mobility O BIT STRING Information related YES ignoreInformation (SIZE (32)) to the handover; the source eNB provides it inorder to enable later analysis of the conditions that led to a wrong HO.Masked IMEISV O 9.2.69 YES ignore UE History O OCTET VisitedCellInfoListYES ignore Information from STRING contained in the the UEUEInformationResponse message (TS 36.331 [9]) Expected UE O 9.2.70 YESignore Behaviour Specific indicator: O YES ignore SeNB UE X2AP IDmeasurement O YES ignore results of SCG (serving) cells, which could becontained in a container specific indicator: O YES ignore SeNB ID orlist of candidate SeNB IDs

Referring to Table 5, comparing with the current Handover Requestmessage shown in Table 1, the specific indicator of SeNB UE X2AP ID maybe added in the Handover Request message. The specific indicator of SeNBUE X2AP ID may indicate that the source MeNB has intention to keep theSeNB service/bearer. Further, the specific indicator of measurementsresults of SCG cells may be added in the Handover Request message.Further, the specific indicator of SeNB ID or list of candidate SeNB IDsmay be added in the Handover Request message.

Upon receiving the Handover Request message including the firstindicator, the target MeNB may know that the SeNB service/bearer shouldbe kept. Then, in step S1510, the target MeNB transmits the SeNBAddition/Modification Request message, which includes a secondindicator, to the SeNB. The second indicator may indicate the SeNB thatthe requested bearers (with IDs) are currently served by the SeNB. Thatis, the second indicator may provide the SeNB with the meaning that theSeNB has to accept all of the corresponding bearers or partially. Thesecond indicator may further include the updated security key, generatedby the target MeNB, for the SeNB bearers.

In other words, the second indicator may help the SeNB to know that theUE was served by the SeNB before handover is triggered. That is, thesecond indicator may indicate the SeNB so that the SeNB can identify theUE. The second indicator may be realized by the old SeNB UE X2AP ID,which was allocated by the SeNB on the source MeNB side during theaddition procedure. In addition, the source MeNB ID may be added in theSeNB Addition Request message in order to help the SeNB to identify theUE. The source MeNB ID may help to avoid the situation that X2AP ID wasimplemented per interface instead of per node.

Table 6 shows an example of the SeNB Addition Request message accordingto an embodiment of the present invention.

TABLE 6 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject MeNB UE X2AP ID M eNB UE Allocated at YES reject X2AP the MeNB ID9.2.24 UE Security C- 9.2.29 YES reject Capabilities ifSCG Bearer OptionSeNB Security Key C- 9.2.72 The S-KeNB YES reject ifSCG which is Bearerprovided by Option the MeNB, see TS 33.401 [18]. SeNB UE Aggregate M UEThe UE YES reject Maximum Bit Rate Aggregate Aggregate Maximum MaximumBit Bit Rate is split Rate into MeNB 9.2.12 UE Aggregate Maximum BitRate and SeNB UE Aggregate Maximum Bit Rate which are enforced by MeNBand SeNB respectively. Serving PLMN O PLMN The serving YES ignoreIdentity PLMN of the 9.2.4 SCG in the SeNB. E-RABs To Be 1 YES rejectAdded List >E-RABs To Be 1 . . . <maxnoof Bearers> EACH reject AddedItem >>CHOICE M Bearer Option >>>SCG Bearer >>>>E-RAB M 9.2.23 — —ID >>>>E-RAB M 9.2.9 Includes — — Level QoS necessary Parameters QoSparameters >>>>DL O 9.2.5 — — Forwarding >>>>S1 UL M GTP SGW — — GTPTunnel Tunnel endpoint of Endpoint Endpoint the S1 9.2.1 transportbearer. For delivery of UL PDUs. >>>Split Bearer >>>>E-RAB M 9.2.23 — —ID >>>>E-RAB M 9.2.9 Includes — — Level QoS necessary Parameters QoSparameters >>>>MeNB M GTP MeNB — — GTP Tunnel Tunnel endpoint ofEndpoint Endpoint the X2 9.2.1 transport bearer. For delivery of ULPDUs. MeNB to SeNB M OCTET Includes the YES reject Container STRING SCG-ConfigInfo message as defined in TS 36.331 [9] Old SeNB UE O It is YESignore X2AP ID allocated by the SeNB on the source MeNB side during theaddition procedure Source MeNB ID O YES ignore

Referring to Table 6, comparing with the current SeNB Addition Requestmessage shown in Table 4, the old SeNB UE X2AP ID and the source MeNB IDmay be added in the SeNB Addition Request message. The old SeNB UE X2APID may correspond to the second indicator. The old SeNB UE X2AP ID maybe allocated by the SeNB on the source MeNB side during the additionprocedure.

In step S1511, the SeNB gives a response with the SeNBAddition/Modification Request Acknowledge message, which includes athird indicator. The third indicator may indicate whether the originalSeNB bearers are all accepted or partially accepted. That is, the thirdindicator may indicate information on which specific SeNB bearer isaccepted and which specific SeNB bearer is not accepted. For the SeNBbearers not accepted by the SeNB, the target MeNB may accept therejected SeNB bearer and put in the accepted MeNB bearers list, which isdescribed in step S1520 below.

In step S1520, the target MeNB transmit the Handover Request Acknowledgemessage, which includes a fourth indicator, to the source MeNB. Thefourth indicator may be indication of SeNB bearers and/or MeNB bearers.The fourth indicator may indicate the accepted SeNB bearers and/orrejected SeNB bearers, which are the SeNB bearers within the requestedlist. The fourth indicator may further indicate the accepted MeNBbearers and/or rejected MeNB bearers. As described above, if therejected SeNB bearer exists, the accepted MeNB bearers may include therejected SeNB bearer. The fourth indicator may further indicate whetherthe request of keeping the SeNB bearers are accepted or partiallyaccepted.

In other words, the fourth indicator may indicate the source MeNBclearly whether the SeNB is kept or not. The fourth indicator mayindicate to the source MeNB for release handling.

Table 7 shows an example of the Handover Request Acknowledge messageaccording to the embodiment of the present invention.

TABLE 7 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESreject Old eNB UE M eNB UE X2AP Allocated at the YES ignore X2AP ID IDsource eNB 9.2.24 New eNB UE M eNB UE X2AP Allocated at the YES ignoreX2AP ID target eNB 9.2.24 E-RABs Admitted 1 YES ignore List > E-RABs 1 .. . <max noof Bearers> EACH ignore Admitted Item >>E-RAB ID M 9.2.23 —— >>UL GTP Tunnel O GTP Tunnel Identifies the X2 — — Endpoint Endpoint9.2.1 transport bearer used for forwarding of UL PDUs >>DL GTP Tunnel OGTP Tunnel Identifies the X2 — — Endpoint Endpoint 9.2.1 transportbearer. used for forwarding of DL PDUs E-RABs Not O E-RAB List A valuefor E-RAB YES ignore Admitted List 9.2.28 ID shall only be present oncein E-RABs Admitted List IE and in E- RABs Not Admitted List IE. TargeteNB To M OCTET Includes the RRC YES ignore Source eNB STRING E-UTRAHandover Transparent Command message Container as defined in subclause10.2.2 in TS 36.331 [9] Criticality O 9.2.7 YES ignore DiagnosticsIndicator of O YES ignore successful SeNB Keeping

Referring to Table 7, comparing with the current Handover RequestAcknowledge message shown in Table 2, the indicator of successful SeNBkeeping is further added. The indicator of successful SeNB keeping maycorrespond to the fourth indicator.

In step S1521, the source MeNB may transmit the SeNB ModificationRequest message or SeNB Release Request message to the SeNB. The SeNBModification Request message may be transmitted when partial SeNBbearers are accepted. The SeNB Release Request message may betransmitted when all SeNB bearers are rejected. The SeNB ModificationRequest message or SeNB Release Request message may include a fifthindicator. The fifth indicator may indicate that each SeNB bearer is tobe kept in the SeNB. The fifth indicator may further indicate that eachSeNB bearer is to be handed over to the target MeNB. The fifth indicatormay further indicate the SeNB that only the X2 interface relatedresources, instead of radio resources, are released in case keeping issuccessful.

In other words, the fifth indicator may indicate to the SeNB for releasehandling. The fifth indicator may clearly indicate the SeNB the bearersto be kept in the SeNB through IP address of the SeNB and GPRS tunnelingprotocol (GTP) tunnel endpoint ID (TEID) allocated by the SeNB and thebearers to be handed over to the target MeNB through IP address of theMeNB and GTP TEID allocated by the MeNB.

Table 8 shows an example of the SeNB Release Request message accordingto the embodiment of the present invention.

TABLE 8 IE type and Semantics Assigned IE/Group Name Presence Rangereference description Criticality Criticality Message Type M 9.2.13 YESignore MeNB UE X2AP M eNB UE X2AP Allocated at the YES reject ID ID MeNB9.2.24 SeNB UE X2AP O eNB UE X2AP Allocated at the YES reject ID ID SeNB9.2.24 Cause O 9.2.6 YES ignore E-RABs To Be 0 . . . 1 — — ReleasedList >E-RABs To 1 . . . <max noof Bearers> EACH ignore Be ReleasedItem >>CHOICE M Bearer Option >>>SCG Bearer >>>>E-RAB ID M 9.2.23 —— >>>>UL O GTP Tunnel Identifies the X2 — — Forwarding GTP Endpoint9.2.1 transport bearer Tunnel Endpoint used for forwarding of ULPDUs >>>>DL O GTP Tunnel Identifies the X2 — — Forwarding GTP Endpoint9.2.1 transport bearer. Tunnel Endpoint used for forwarding of DLPDUs >>>Split Bearer >>>>E-RAB ID M 9.2.23 — — >>>>DL O GTP TunnelIdentifies the X2 — — Forwarding GTP Endpoint 9.2.1 transport bearer.Tunnel Endpoint used for forwarding of DL PDUs Indicator of O YES ignorereleasing only X2 related resource

Referring to Table 8, comparing with the current Handover RequestAcknowledge message shown in Table 3, the indicator of releasing only X2related resource is further added. The indicator of releasing only X2related resource may correspond to the fifth indicator.

Even though it is not described in FIG. 15, but as described in FIG. 11,the source MeNB may transmit the RRCConnectionReconfiguration message tothe UE with the updated MeNB and SeNB configuration. And, the randomaccess may be performed, and the UE may transmit theRRCConnectionReconfigurationComplete message to the target MeNB.

In step S1530, the target MeNB transmits the SeNB ReconfigurationComplete message to the SeNB for confirmation.

In step S1540, the SeNB may transmit the SN Status Transfer message tothe source MeNB for the partial SeNB bearers (with indicator), for whichthe data forwarding is needed. In step S1541, the source MeNB maytransmit the SN Status Transfer message to the target MeNB for the MeNBbearers (with indicator), and also for the partial SeNB bearers of stepS1540. In step S1542, data forwarding may be initiated for the MeNBbearers and the partial SeNB bearers.

In step S1550, the target MeNB may transmit the Path Switch Requestmessage or E-RAB Modification Indication message to the MME for bearerpath change. The Path Switch Request message or E-RAB ModificationIndication message may include an indication of E-RABs to be modifiedand E-RABs not to be modified. The Path Switch Request message or E-RABModification Indication message may further include an indication ofE-RABs for the MeNB and E-RABs for the SeNB which keeps the SeNBbearers.

In step S1560, the S-GW and MME may perform bearer modification forbearer information updates by exchanging the Modify BearerRequest/Response messages. The Modify Bearer Request/Response messagesmay include an indication of bearers to be modified and bearers not tobe modified.

In step S1570, the end marker may be transmitted. In step S1571, the newpackets may be transmitted.

In step S1580, the MME may transmit the Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message to the target MeNBfor response of bearer path change. The Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message may include anindication of E-RABs modified and E-RABs failed to modify. The PathSwitch Request Acknowledge message or E-RAB Modification Confirmationmessage may further include information of MeNB bearers or SeNB bearers.

In step S1590, the target MeNB transmits the UE Context Release messageto the source MeNB to release the UE context.

In step S1591, the source MeNB may transmit the UE Context Releasemessage to the SeNB if all of the SeNB bearers are rejected in stepS1511 described above.

FIG. 16 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention. In thisembodiment, it may be assumed that the SeNB bearers (or, SeNB E-RABs)are kept during the handover. At first, the UE (not described in FIG.16) may transmit the measurement report to the source MeNB, which makesa handover decision to the target MeNB for the UE. The source MeNB mayalso know that the UE is receiving a certain service from the SeNB,which is commonly controlled by the source MeNB and the target MeNB.

In step S1600, the source MeNB transmits the Handover Request message,which includes a first indicator, to target MeNB. The first indicatormay be indication of SeNB bearers and/or MeNB bearers. By the firstindicator, the target MeNB may differentiate the E-RABs To Be Setup Listfor the SeNB (i.e. SeNB bearers) and the E-RABs To Be Setup List for theMeNB (i.e. MeNB bearers). Further, by the first indicator, the targetMeNB may know that the source MeNB has intention to keep the SeNBservice/bearer. For example, the first indicator may be an indicator ofthe SeNB, e.g. (cell) ID of the SeNB. In this case, the first indicatormay indicate the target MeNB which SeNB service/bearer is kept for thisthe UE. The Handover Request message may follow Table 5 described above.

Upon receiving the Handover Request message including the firstindicator, the target MeNB may know that the SeNB service/bearer shouldbe kept. Then, in step S1610, the target MeNB transmits the SeNBAddition/Modification Request message, which includes a secondindicator, to the SeNB. The second indicator may indicate the SeNB thatthe requested bearers (with IDs) are currently served by the SeNB. Thatis, the second indicator may provide the SeNB with the meaning that theSeNB has to accept the corresponding bearers. The second indicator mayfurther include the updated security key, generated by the target MeNB,for the SeNB bearers. The SeNB Addition Request message may follow Table6 described above.

In step S1611, the SeNB gives a response with the SeNBAddition/Modification Request Acknowledge message, which includes athird indicator. The third indicator may indicate whether the originalSeNB bearers are accepted or not.

In step S1620, the target MeNB transmit the Handover Request Acknowledgemessage, which includes a fourth indicator, to the source MeNB. Thefourth indicator may be indication of SeNB bearers and/or MeNB bearers.The fourth indicator may indicate the accepted SeNB bearers, which arethe SeNB bearers within the requested list. The fourth indicator mayfurther indicate the accepted MeNB bearers and/or rejected MeNB bearers.The fourth indicator may further indicate whether the request of keepingthe SeNB bearers are accepted or not. The Handover Request Acknowledgemessage may follow Table 7 described above.

Even though it is not described in FIG. 16, but as described in FIG. 11,the source MeNB may transmit the RRCConnectionReconfiguration message tothe UE with the updated MeNB and SeNB configuration. And, the randomaccess may be performed, and the UE may transmit theRRCConnectionReconfigurationComplete message to the target MeNB.

In step S1630, the target MeNB transmits the SeNB ReconfigurationComplete message to the SeNB for confirmation.

In step S1640, the source MeNB may transmit the SN Status Transfermessage to the target MeNB only for the MeNB bearers (with indicator).Since the SeNB bearers are kept, data forwarding may not be needed forthe SeNB bearers. In step S1641, data forwarding may be initiated forthe MeNB bearers.

In step S1650, the target MeNB may transmit the Path Switch Requestmessage or E-RAB Modification Indication message to the MME for bearerpath change. The Path Switch Request message or E-RAB ModificationIndication message may include an indication of E-RABs to be modifiedand E-RABs not to be modified. The Path Switch Request message or E-RABModification Indication message may further include an indication ofE-RABs for the MeNB and E-RABs for the SeNB which keeps the SeNBbearers.

In step S1660, the S-GW and MME may perform bearer modification forbearer information updates by exchanging the Modify BearerRequest/Response messages. The Modify Bearer Request/Response messagesmay include an indication of bearers to be modified and bearers not tobe modified.

In step S1670, the end marker may be transmitted. In step S1671, the newpackets may be transmitted.

In step S1680, the MME may transmit the Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message to the target MeNBfor response of bearer path change. The Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message may include anindication of E-RABs modified and E-RABs failed to modify. The PathSwitch Request Acknowledge message or E-RAB Modification Confirmationmessage may further include information of MeNB bearers or SeNB bearers.

In step S1690, the target MeNB transmits the UE Context Release messageto the source MeNB to release the UE context.

FIG. 17 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention.

In step S1700, the source MeNB starts the handover procedure byinitiating the X2 handover preparation procedure. The source MeNBincludes the SCG configuration in the HandoverPreparationInformation.The source MeNB includes the SeNB UE X2AP ID and SeNB ID as a referenceto the UE context in the SeNB that was established by the source MeNB inthe Handover Request message. The handover request message may followTable 5 described above.

In step S1700, if the target MeNB decides to keep the SeNB, the targetMeNB sends SeNB Addition Request message to the SeNB including the SeNBUE X2AP ID as a reference to the UE context in the SeNB that wasestablished by the source MeNB. The SeNB Addition Request message mayfollow Table 6 described above.

In step S1711, the SeNB replies with SeNB Addition Request Acknowledgemessage to the target MeNB. An indicator as the UE context kept in theSeNB may be introduced in the SeNB Addition Request Acknowledge message.

In step S1720, the target MeNB includes within the Handover RequestAcknowledge message a transparent container to be sent to the UE as anRRC message to perform the handover which also includes the SCGconfiguration, and may also provide forwarding addresses to the sourceMeNB. The target MeNB indicates to the source MeNB that the UE contextin the SeNB is kept if the target MeNB and the SeNB decided to keep theUE context in the SeNB in step S1710 and S1711. The Handover RequestAcknowledge message may follow Table 7 described above.

In step S1730, the source MeNB sends the SeNB Release Request message tothe SeNB. The source MeNB indicates to the SeNB that the UE context inSeNB is kept. If the indication as the UE context kept in SeNB isincluded, the SeNB keeps the UE context. The SeNB Release Requestmessage may follow Table 8 described above.

In step S1740, the source MeNB triggers the UE to apply the newconfiguration.

In steps S1741 and S1742, the UE synchronizes to the target MeNB andreplies with RRCConnectionReconfigurationComplete message.

In step S1743, the UE synchronizes to the SeNB.

In step S1750, if the RRC connection reconfiguration procedure wassuccessful, the target MeNB informs the SeNB.

In steps S1760 and S1761, data forwarding from the source MeNB takesplace. Data forwarding may be omitted for SCG bearers. Direct dataforwarding from the source MeNB to the SeNB is not possible for splitbearers. Direct data forwarding may occur only for bearer type change.

From steps S1770 to S1773, the target MeNB initiates the S1 path switchprocedure. If new UL TEIDs of the S-GW are included, the target MeNBperforms MeNB initiated SeNB modification procedure to provide them tothe SeNB.

In step S1780, the target MeNB initiates the UE context releaseprocedure towards the source MeNB.

In step S1781, upon reception of the UE context release message, theSeNB can release C-plane related resource associated to the UE contexttowards the source MeNB. Any ongoing data forwarding may continue. TheSeNB shall not release the UE context associated with the target MeNB ifthe indication was included in the SeNB Release Request in step S1730.

FIG. 18 shows a method for performing, by a target MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention. A UE is connected to both the source MeNB and the SeNBcurrently, and the UE is to be handed over to the target MeNB by thehandover procedure without change of the SeNB.

In step S1800, the target MeNB decides to keep bearers of the SeNBduring inter-MeNB handover without SeNB change. In step S1810, thetarget MeNB transmits a handover request acknowledge message includingan indication of keeping the bearers of the SeNB to the source MeNB.Step S1810 may correspond to step S1520 in FIG. 15, step S1620 in FIG.16, or step S1720 in FIG. 17. That is, the handover request acknowledgemessage may follow Table 7 described above. The indication of keepingthe bearers of the SeNB indicates that a UE context in the SeNB is kept.The indication of keeping the bearers of the SeNB may indicate whether arequest of keeping the bearers of the SeNB are accepted or not. Theindication of keeping the bearers of the SeNB may indicate whether theSeNB is kept or not.

The target MeNB may further receive a handover request message from thesource MeNB before deciding to keep the bearers of the SeNB. Thehandover request message may include an ID of the SeNB.

FIG. 19 shows a method for performing, by a source MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention. A UE is connected to both the source MeNB and the SeNBcurrently, and the UE is to be handed over to the target MeNB by thehandover procedure without change of the SeNB.

In step S1900, the source MeNB receives a handover request acknowledgemessage including a first indication of keeping bearers of the SeNB froma target MeNB in dual connectivity. Step S1900 may correspond to stepS1520 in FIG. 15, step S1620 in FIG. 16, step S1720 in FIG. 17, or stepS1810 in FIG. 18. That is, the handover request acknowledge message mayfollow Table 7 described above. The first indication of keeping thebearers of the SeNB may indicate that a UE context in the SeNB is kept.

In step S1910, the source MeNB transmits a SeNB release request messageincluding a second indication of keeping the bearers of the SeNB to theSeNB. Step S1910 may correspond to step S1521 in FIG. 15, or step S1730in FIG. 17. That is, the SeNB release request message may follow Table 8described above. The second indication of keeping the bearers of theSeNB may indicate that a UE context in the SeNB is kept. The secondindication of keeping the bearers of the SeNB may further indicate thatonly X2 interface related resources are released when the UE context inthe SeNB is kept. The second indication of keeping the bearers of theSeNB may further indicate that the bearers in the SeNB is kept throughan IP address of the SeNB and a GTP TEID allocated by the SeNB. Uponreceiving the SeNB release request message, the SeNB shall, ifsupported, only release the resources related to the UE-associatedsignaling connection between the MeNB and the SeNB.

The source MeNB may further transmit a handover request message to thetarget MeNB before receiving the handover request acknowledge message.The handover request message may include an ID of the SeNB.

FIG. 20 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention. A UE isconnected to both the source MeNB and the SeNB currently, and the UE isto be handed over to the target MeNB by the handover procedure withoutchange of the SeNB.

In step S2000, the target MeNB decides to keep bearers of the SeNBduring inter-MeNB handover without SeNB change. In step S2010, thetarget MeNB transmits a handover request acknowledge message including afirst indication of keeping the bearers of the SeNB to the source MeNB.Step S2010 may correspond to step S1520 in FIG. 15, step S1620 in FIG.16, step S1720 in FIG. 17, step S1810 in FIG. 18, or step S1900 in FIG.19. That is, the handover request acknowledge message may follow Table 7described above. The indication of keeping the bearers of the SeNBindicates that a UE context in the SeNB is kept. The indication ofkeeping the bearers of the SeNB may indicate whether a request ofkeeping the bearers of the SeNB are accepted or not. The indication ofkeeping the bearers of the SeNB may indicate whether the SeNB is kept ornot.

In step S2020, the source MeNB transmits a SeNB release request messageincluding a second indication of keeping the bearers of the SeNB to theSeNB. Step S2020 may correspond to step S1521 in FIG. 15, step S1730 inFIG. 17, or step S1910 in FIG. 19. That is, the SeNB release requestmessage may follow Table 8 described above. The second indication ofkeeping the bearers of the SeNB may indicate that a UE context in theSeNB is kept. The second indication of keeping the bearers of the SeNBmay further indicate that only X2 interface related resources arereleased when the UE context in the SeNB is kept. The second indicationof keeping the bearers of the SeNB may further indicate that the bearersin the SeNB is kept through an IP address of the SeNB and a GTP TEIDallocated by the SeNB. Upon receiving the SeNB release request message,the SeNB shall, if supported, only release the resources related to theUE-associated signaling connection between the MeNB and the SeNB.

FIG. 21 shows a method for performing, by a source MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention. A UE is connected to both the source MeNB and the SeNBcurrently, and the UE is to be handed over to the target MeNB by thehandover procedure without change of the SeNB.

In step S2100, the source MeNB transmits a handover request messageincluding an indication of the SeNB to the target MeNB. Step S2100 maycorrespond to step S1500 in FIG. 15, step S1600 in FIG. 16, or stepS1700 in FIG. 17. That is, the handover request message may follow Table5 described above. The indication of the SeNB corresponds to a SeNB ID.Or, the indication of the SeNB may correspond to a list of candidateSeNB IDs. Further, the indication of the SeNB may correspond to a SeNBUE X2AP ID, which was allocated by the SeNB. The SeNB UE X2AP ID may beallocated by the SeNB before the handover procedure is triggered. Thehandover request message may further include measurement results of SCGcells for the SeNB.

In step S2110, the source MeNB receives a handover request acknowledgemessage from the target MeNB. The handover request acknowledge messagemay include an indication of keeping bearers of the SeNB. Step S2110 maycorrespond to step S1520 in FIG. 15, step S1620 in FIG. 16, step S1720in FIG. 17, step S1810 in FIG. 18, step S1900 in FIG. 19, or step S2010in FIG. 20. That is, the handover request acknowledge message may followTable 7 described above.

FIG. 22 shows a method for performing, by a target MeNB, an inter-MeNBhandover without SeNB change according to an embodiment of the presentinvention. A UE is connected to both the source MeNB and the SeNBcurrently, and the UE is to be handed over to the target MeNB by thehandover procedure without change of the SeNB.

In step S2200, the target MeNB receives a handover request messageincluding a first indication of the SeNB from the source MeNB. StepS2200 may correspond to step S1500 in FIG. 15, step S1600 in FIG. 16,step S1700 in FIG. 17, or step S2100 in FIG. 21. That is, the handoverrequest message may follow Table 5 described above. The first indicationof the SeNB may correspond to a SeNB ID.

In step S2210, the target MeNB transmits a SeNB addition request messageincluding a second indication of a UE, served by the SeNB, to the SeNB.Step S2210 may correspond to step S1510 in FIG. 15, step S1610 in FIG.16, or step S1710 in FIG. 17. That is, the SeNB addition request messagemay follow Table 6 described above. The second indication of the UE maycorrespond to an old SeNB UE X2AP ID, which was allocated by the SeNB.The SeNB addition request message may further include a source MeNB ID.

The target MeNB may further receive a SeNB addition acknowledge messageincluding a third indication of keeping bearers of the SeNB from theSeNB. The SeNB addition acknowledge message may correspond to step S1511in FIG. 15, step S1611 in FIG. 16, or step S1711 in FIG. 17. The thirdindication of keeping bearers of the SeNB may indicate which specificSeNB bearer is accepted or not.

FIG. 23 shows a method for performing an inter-MeNB handover withoutSeNB change according to an embodiment of the present invention. A UE isconnected to both the source MeNB and the SeNB currently, and the UE isto be handed over to the target MeNB by the handover procedure withoutchange of the SeNB.

In step S2300, the source MeNB transmits a handover request messageincluding a first indicator of the SeNB to the target MeNB. Step S2300may correspond to step S1500 in FIG. 15, step S1600 in FIG. 16, stepS1700 in FIG. 17, step S2100 in FIG. 21, or step S2200 in FIG. 22. Thatis, the handover request message may follow Table 5 described above. Thefirst indicator of the SeNB corresponds to a SeNB ID. Or, the firstindicator of the SeNB may correspond to a list of candidate SeNB IDs.Further, the first indicator of the SeNB may correspond to a SeNB UEX2AP ID, which was allocated by the SeNB. The SeNB UE X2AP ID may beallocated by the SeNB before the handover procedure is triggered.

In step S2310, the target MeNB transmits a SeNB addition request messageincluding a second indicator of a UE, served by the SeNB, to the SeNB.Step S2310 may correspond to step S1510 in FIG. 15, step S1610 in FIG.16, step S1710 in FIG. 17, or step S2210 in FIG. 22. That is, the SeNBaddition request message may follow Table 6 described above. The secondindicator of the UE may correspond to an old SeNB UE X2AP ID, which wasallocated by the SeNB.

In step S2320, the SeNB transmits a SeNB addition acknowledge messageincluding a third indicator of whether bearers of the SeNB are allaccepted or partially accepted to the target MeNB. The SeNB additionacknowledge message may correspond to step S1511 in FIG. 15, step S1611in FIG. 16, or step S1711 in FIG. 17.

FIG. 24 shows an example of a deployment scenario with two SeNBs and twoMeNBs. Referring to FIG. 24, SeNB 1 belongs to MeNB 1, and SeNB 2belongs to MeNB 2. A UE is receiving dual connectivity service viabearer 1 from MeNB 1, and via bearer 2 from SeNB 1. The UE may be handedover to the neighbor MeNB, i.e. MeNB 2. If the current X2 handoverprocedure shown in FIG. 11 and the current SeNB addition procedure shownin FIG. 12 are applied to the deployment scenario shown in FIG. 24, itis not optimized since the target MeNB has to add the SeNB again afterX2 handover procedure. In this way, the path switch message to the MMEand RRC signaling to the UE have to be transmitted twice.

FIG. 25 shows a method for performing an inter-MeNB handover accordingto an embodiment of the present invention. This embodiment is to solvethe problem described in the deployment scenario in FIG. 24. In thisembodiment, direction addition to the neighbor SeNB during the handovermay be assumed. At first, the UE may transmit the measurement report tothe source MeNB, which makes a handover decision to the target MeNB forthe UE.

In step S2500, the source MeNB transmits the Handover Request message,which includes a first indicator, to target MeNB. The first indicatormay be indication of SeNB bearers and/or MeNB bearers. By the firstindicator, the target MeNB may differentiate the E-RABs To Be Setup Listfor the SeNB (i.e. SeNB bearers) and the E-RABs To Be Setup List for theMeNB (i.e. MeNB bearers). Further, the first indicator may be anindicator of the SeNB to indicate the target MeNB to add directly. Inthis case, the first indicator may be a SeNB (cell) ID. Or, the firstindicator may be a list of potential candidate SeNBs for the target MeNBto select. Further, the Handover Request message may include assistantinformation, i.e. measurement information, for the target MeNB to selectthe target SeNB. The measurement information may be contained in anindependent IE in the Handover Request message. For example, themeasurement information may be contained in MeNB to SeNB Containerincluding SCG-ConfigInfo, as measResultServCellListSCG indicatingmeasurement results of SCG (serving) cells. Or, the measurementinformation may be contained in RRC Context IE in the Handover RequestMessage. The Handover Request message may follow Table 5 describedabove.

Upon receiving the Handover Request message including the firstindicator, the target MeNB may know that the SeNB service/bearer may beadded directly. Then, in step S2510, the target MeNB transmits the SeNBAddition Request message, which includes a second indicator, to thetarget SeNB. The second indicator may indicate the target SeNB bearersfor the SeNB. The second indicator may further include the updatedsecurity key, generated by the target MeNB, for the SeNB bearers.

In step S2511, the target SeNB gives a response with the SeNB AdditionRequest Acknowledge message, which includes a third indicator. The thirdindicator may indicate whether the SeNB bearers are all accepted orpartially accepted. That is, the third indicator may indicateinformation on which specific SeNB bearer is accepted and which specificSeNB bearer is not accepted. For the SeNB bearers not accepted by theSeNB, the target MeNB may accept the rejected SeNB bearer and put in theaccepted MeNB bearers list, which is described in step S2520 below.

In step S2520, the target MeNB transmit the Handover Request Acknowledgemessage, which includes a fourth indicator, to the source MeNB. Thefourth indicator may be indication of SeNB bearers and/or MeNB bearers.The fourth indicator may indicate the accepted SeNB bearers and/orrejected SeNB bearers, which are the SeNB bearers within the requestedlist. The fourth indicator may further indicate the accepted MeNBbearers and/or rejected MeNB bearers. As described above, if therejected SeNB bearer exists, the accepted MeNB bearers may include therejected SeNB bearer. The fourth indicator may further indicate directdata forwarding or indirect data forwarding.

In step S2530, the source MeNB may transmit the SeNB Release Requestmessage to the source SeNB. The SeNB Release Request message may includea fifth indicator. The fifth indicator may indicate that each SeNBbearer is to be handed over to the target MeNB and target SeNB. Thefifth indicator may further indicate direct data forwarding or indirectdata forwarding.

In step S2540, the source MeNB transmits theRRCConnectionReconfiguration message to the UE with the updated MeNB andSeNB configuration, which may include the target MeNB configurationand/or target SeNB configuration.

In step S2541 and step S2542, the random access may be performed to thetarget MeNB and target SeNB, and the following will beRRCConnectionReconfigurationComplete message.

In step S2550, the target MeNB transmits the SeNB ReconfigurationComplete message to the target SeNB for confirmation.

In step S2560, the source SeNB may transmit the SN Status Transfermessage to the source MeNB for the bearers that data forwarding isneeded.

In step S2561 and step S2562, the source MeNB may transmit the SN StatusTransfer message to the target MeNB, and partial may be forwarded to thetarget SeNB.

In step S2563, data forwarding may be initiated for the bearers in stepsS2560, S2561 and S2562. Direct data forwarding may also be possiblebased on the indication.

In step S2570, the target MeNB may transmit the Path Switch Requestmessage or E-RAB Modification Indication message to the MME for bearerpath change. The Path Switch Request message or E-RAB ModificationIndication message may include an indication of E-RABs to be modifiedand E-RABs not to be modified. The Path Switch Request message or E-RABModification Indication message may further include an indication ofE-RABs for the MeNB and E-RABs for the SeNB which keeps the SeNBbearers.

In step S2571, the S-GW and MME may perform bearer modification forbearer information updates by exchanging the Modify BearerRequest/Response messages. The Modify Bearer Request/Response messagesmay include an indication of bearers to be modified and bearers not tobe modified. The Modify Bearer Request/Response messages may furtherinclude an indication of E-RABs for the MeNB and E-RABs for the SeNBwhich keeps the SeNB bearers

In step S2572, the end marker may be transmitted. In step S2573, the newpackets may be transmitted.

In step S2580, the MME may transmit the Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message to the target MeNBfor response of bearer path change. The Path Switch Request Acknowledgemessage or E-RAB Modification Confirmation message may include anindication of E-RABs modified and E-RABs failed to modify. The PathSwitch Request Acknowledge message or E-RAB Modification Confirmationmessage may further include information of MeNB bearers or SeNB bearers.

In step S2590, the target MeNB transmits the UE Context Release messageto the source MeNB to release the UE context.

In step S2591, the source MeNB may transmit the UE Context Releasemessage to the source SeNB.

FIG. 26 shows a wireless communication system to implement an embodimentof the present invention.

A first eNB 800 includes a processor 810, a memory 820, and atransceiver 830. The first eNB 800 may be one of a source/target MeNB orsource/target SeNB. The processor 810 may be configured to implementproposed functions, procedures, and/or methods in this description.Layers of the radio interface protocol may be implemented in theprocessor 810. The memory 820 is operatively coupled with the processor810 and stores a variety of information to operate the processor 810.The transceiver 830 is operatively coupled with the processor 810, andtransmits and/or receives a radio signal.

A second eNB 900 includes a processor 910, a memory 920 and atransceiver 930. The second eNB 900 may be one of a source/target MeNBor source/target SeNB. The processor 910 may be configured to implementproposed functions, procedures and/or methods described in thisdescription. Layers of the radio interface protocol may be implementedin the processor 910. The memory 920 is operatively coupled with theprocessor 910 and stores a variety of information to operate theprocessor 910. The transceiver 930 is operatively coupled with theprocessor 910, and transmits and/or receives a radio signal.

The processors 810, 910 may include application-specific integratedcircuit (ASIC), other chipset, logic circuit and/or data processingdevice. The memories 820, 920 may include read-only memory (ROM), randomaccess memory (RAM), flash memory, memory card, storage medium and/orother storage device. The transceivers 830, 930 may include basebandcircuitry to process radio frequency signals. When the embodiments areimplemented in software, the techniques described herein can beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. The modules can be stored inmemories 820, 920 and executed by processors 810, 910. The memories 820,920 can be implemented within the processors 810, 910 or external to theprocessors 810, 910 in which case those can be communicatively coupledto the processors 810, 910 via various means as is known in the art.

In view of the exemplary systems described herein, methodologies thatmay be implemented in accordance with the disclosed subject matter havebeen described with reference to several flow diagrams. While forpurposed of simplicity, the methodologies are shown and described as aseries of steps or blocks, it is to be understood and appreciated thatthe claimed subject matter is not limited by the order of the steps orblocks, as some steps may occur in different orders or concurrently withother steps from what is depicted and described herein. Moreover, oneskilled in the art would understand that the steps illustrated in theflow diagram are not exclusive and other steps may be included or one ormore of the steps in the example flow diagram may be deleted withoutaffecting the scope and spirit of the present disclosure.

1. (canceled)
 2. A method performed by a target master node (MN) in aninter-MN handover procedure, the method comprising: receiving a handoverrequest message from a source MN, wherein the handover request messageincludes an identifier (ID) of a secondary node (SN); transmitting a SNaddition request message to the SN, wherein the SN addition requestmessage includes an ID of a user equipment (UE), wherein the ID of theUE was allocated by the SN; receiving a SN addition request acknowledgemessage from the SN as a response to the SN addition request message;transmitting a handover request acknowledge message to the source MN asa response to the handover request message, wherein the handover requestacknowledge message includes information indicating that the SN is keptin the inter-MN handover procedure, wherein the UE is currentlyconnected to both the source MN and the SN in dual connectivity.
 3. Themethod of claim 2, wherein the SN addition request message includes anID of the source MN.
 4. The method of claim 2, wherein the SN additionrequest message includes a security key for the SN.
 5. The method ofclaim 2, wherein the ID of the UE is a SeNB UE X2AP ID.
 6. The method ofclaim 2, wherein the SN addition request acknowledge message includesinformation on which SN bearers are accepted or not to be kept by the SNfor the UE.
 7. The method of claim 2, wherein the target MN is a targetmaster eNodeB (MeNB), wherein the source MN is a source MeNB, andwherein the SN is a SeNB.
 8. A target master node (MN) in an inter-MNhandover procedure, the target MN comprising: a memory; a transceiver;and a processor, operably coupled to the memory and the transceiver,that: controls the transceiver to receive a handover request messagefrom a source MN, wherein the handover request message includes anidentifier (ID) of a secondary node (SN); controls the transceiver totransmit a SN addition request message to the SN, wherein the SNaddition request message includes an ID of a user equipment (UE),wherein the ID of the UE was allocated by the SN, controls thetransceiver to receive a SN addition request acknowledge message fromthe SN as a response to the SN addition request message; controls thetransceiver to transmit a handover request acknowledge message to thesource MN as a response to the handover request message, wherein thehandover request acknowledge message includes information indicatingthat the SN is kept in the inter-MN handover procedure, wherein the UEis currently connected to both the source MN and the SN in dualconnectivity.
 9. The target MN of claim 8, wherein the SN additionrequest message includes an ID of the source MN.
 10. The target MN ofclaim 8, wherein the SN addition request message includes a security keyfor the SN.
 11. The target MN of claim 8, wherein the ID of the UE is aSeNB UE X2AP ID.
 12. The target MN of claim 8, wherein the SN additionrequest acknowledge message includes information on which SN bearers areaccepted or not to be kept by the SN for the UE.
 13. The target MN ofclaim 8, wherein the target MN is a target master eNodeB (MeNB), whereinthe source MN is a source MeNB, and wherein the SN is a SeNB.